Described below is a method and to a system for enabling local switching in mobile radio networks and, more particularly, for enabling local switching of circuit switched (CS) calls in mobile radio networks, in particular GERAN networks.
A GERAN network includes an access network, made of several Base Station Subsystems (BSS), and a core network, made of—in the CS domain—several Mobile Switching Centers (MSC). A BSS includes a Base Station Controller (BSC) connected to several Base Transceiver Stations (BTS). A MSC can optionally be split in a MSC Server (MSC-S), dealing with the Control Plane procedures, and a Media Gateway (MGW), dealing with User Plane data. One BSC is typically connected to a MSC via the A interface, but possibly also to more than one, e.g. for redundancy reasons. One MSC is typically connected to more than one BSC. The A interface is the interface that connects the BSS and the MSC (as described in 3GPP TS 48.008, “Mobile Switching Centre-Base Station System (MSC-BSS) interface”).
In a mobile radio network like GERAN every CS call needs to go through a MSC, including both the signalling and the user plane data flow. This applies also in case the user originating the call and the user terminating the call are connected to BTS's controlled by the same BSC, i.e. also for local calls inside one BSS. This is needed because when a mobile station originates the call, the BSC has no information about the identity of the called party, and whether it is located in the same BSS area or not. The call is always controlled by the MSC that also establishes a circuit (identified by a Circuit Identity Code—CIC) towards the BSS where the call is originated and another circuit (with another CIC) towards the BSS where the call is terminated. In case the calling and the called parties belong to the same BSS, two circuits are established between the BSS and the MSC.
The term local switching refers to the possibility to exclude the MSC from the user plane data flow (or exclude the MGW, when the MSC functionalities are split into a MSC-S and a MGW) whenever a CS call involves two mobile stations served by the same BSS, and rely on the BSC for switching the CS call through to the target cell. One of the important benefits of this is that establishing two circuits on the user plane interface between the BSC and the MSC (or the between the BSC and the MGW) could be avoided and a lot of bandwidth could be saved.
As already described, local switching is currently not possible because, even after the call is established by the MSC, the BSC has no information to correlate “who is talking to whom”. And even if the BSC had the information, the circuits towards the MSC (or MGW) would already be established and could not be released.
A feasibility study is currently ongoing in the 3GPP TSG GERAN committee to define an IP-based A interface in the user plane (see G2-070305, “AoIP Motivation, Architecture, Functional Impacts”, GERAN WG2#35bis and G2-070359, “Draft Techinal Report: A-interface over IP”, GERAN WG2#35bis).
The main idea of such proposal is to replace the user plane circuits between the MSC (or MGW) and the BSC with IP connections (i.e. with a pair of IP addresses and UDP ports) and consequently replace the CICs with identifiers of the IP connection (e.g. with “A over IP Call Instance Codes”) or, equivalently, with identifiers of the signalling connection associated to the it (e.g. with “A over IP Signalling Connection Identifiers”).
The standardization of the A interface over IP (in the User Plane) has many goals, for instance an easier network configuration, and the possibility to use compressed speech codecs between the BSS and the MSC, so that the need of bandwidth on the A interface is reduced.
However, for local CS calls (i.e. calls involving two mobile stations controlled by the same BSC) it is still needed to consume some bandwidth on the A interface. Only with a local switching approach it is possible to bring the bandwidth needed on the A interface down to zero.
Moreover, if local switching is enabled also the end to end delay can be reduced because the transmission path is shortened (it is not necessary to go back and forth to the MSC (MGW)).